This project is a part of a continuing program to use affinity chromatography to determine quantitative parameters for macromolecule-involved interactions. For one model used, ribonuclease and a specific affinity matrix, competitive elutions with soluble ligand were used to measure equilibrium binding constants for ribonuclease and active site ligands. A detailed study was made of the behaviour for the inactive semisynthetic ribonuclease-S' variant containing 4-F-histidine at position 12. This inactive species was shown to bind substrates and inhibitors with a specificity related to that for native ribonuclease. The substrate binding property and quantitative aspects of inhibitor binding can be correlated with the participation of His 12 in substrate binding and catalysis in native ribonuclease. The ribonuclease affinity chromatography system also was used to investigate the possibility of measuring rate constants for protein -immobilized ligand interactions. A project also has been initiated into the use of quantitative affinity chromatography to study multivalently interacting proteins, using a series of phosophorylcholine-specific myeloma IgA proteins as objects for study.